Implantable medical electrical stimulation and/or sensing leads are well known in the fields of cardiac stimulation and monitoring, including cardiac pacing and cardioversion/defibrillation, and in other fields of electrical stimulation or monitoring of electrical signals or other physiologic parameters of the body. A pacemaker or cardioverter/defibrillator implantable pulse generator (IPG) or a cardiac monitor is typically coupled to the heart through one or more of such endocardial leads. The proximal end of such leads typically is formed with a connector, which connects to a terminal of the IPG or monitor. The lead body typically comprises one or more insulated, conductive wire surrounded by an insulating outer sleeve. Each conductive wire couples a proximal lead connector element with a distal stimulation and/or sensing electrode. An endocardial cardiac lead having a single stimulation and/or sensing electrode at the distal lead end and a single conductive wire is referred to as a unipolar lead. An endocardial cardiac lead having two or more stimulation and/or sensing electrodes at the distal lead end and two or more conductive wires is referred to as a bipolar lead or a multi-polar lead, respectively.
In order to implant an endocardial lead within a heart chamber, a transvenous approach is utilized wherein the lead is inserted into and passed through a pathway comprising the subclavian, jugular, or cephalic vein and through the superior vena cava into the right atrium or ventricle. It is necessary to accurately position the sense and/or stimulation electrode surface against the endocardium or within the myocardium at the desired site in order to achieve reliable sensing of the cardiac electrogram and/or to apply stimulation that effectively paces or cardioverts the heart chamber. The desired heart sites include the right atrium, typically the right atrial appendage, the right ventricle, typically the ventricular apex, and the coronary sinus and great vein descending therefrom.
The implantable cardiac lead conductor that has been typically employed in pacing leads or in early cardioversion/defibrillation leads is a single coiled wire or a multi-filar wire coil used alone in the unipolar lead configuration or used in a pair, co-axially arranged and insulated from one another, in a bipolar lead configuration. The coiled wires of such lead conductors may be formed of a single conductive metal or alloy material, e.g. MP35N alloy. Or the coiled wires have been formed of a composite conductive material, typically a silver core wire clad with MP35N alloy or surgical grade stainless steel or the like in a drawn brazed stranded (DBS) or drawn filled tube (DFT) fabrication process well known in the art, to provide increased conductivity while reducing the wire cross-section. An exemplary multi-filar wire coil as shown in commonly assigned U.S. Pat. No. 5,007,435, incorporated herein by reference.
Over the years many improvements have been made in lead bodies in the effort to include more than two lead conductors capable of carrying more current, and to make the lead body diameters smaller, more flexible, and more resistant to fracture. In the implantation of a cardiac device of the types listed above, and in the replacement of previously implanted cardiac leads, two or more transvenous cardiac leads are typically introduced through the venous system into the right chambers or coronary sinus of the heart. It has long been desired to minimize the diameter of the transvenous cardiac lead body to facilitate the introduction of several cardiac leads by the same transvenous approach. Moreover, a number of multi-polar, endocardial cardiac leads have been designed to accommodate more than two electrodes or to make electrical connection with other components, e.g., blood pressure sensors, temperature sensors, pH sensors, or the like, in the distal portion of the lead. In addition, endocardial cardioversion/defibrillation leads were developed for unipolar or bipolar pacing and sensing functions and for delivering cardioversion/defibrillation shocks to a heart chamber intended to be implanted in a heart chamber or a cardiac blood vessel, e.g., the coronary sinus.
The increased number of separate polarity and insulated coiled wire conductors is difficult to accommodate in the conventional coaxial coiled wire conductor winding arrangement having a desired, small, lead body outer diameter. One approach involved the use of separately insulated, coiled wire conductors that are parallel-wound with a common diameter and are separately coupled between a proximal connector element and to a distal electrode or terminal.
It has also been proposed to diminish the lead body diameter further by eliminating the lumen for receiving the stiffening stylet and by replacing the large diameter coiled wire conductors with highly conductive stranded filament wires or cables formed of a plurality of such wires. In bipolar or multi-polar leads, each such wire or cable extends through a separate lumen extending in parallel within a lead body sheath that maintains electrical isolation between them. Examples of such lead body insulating sheaths formed to enclose a plurality of straight, typically stranded, wire lead conductors, miniaturized coiled wire conductors or combinations of such straight and coiled wire conductors are disclosed in U.S. Pat. Nos. 4,608,986, 5,324,321, 5,545,203, and 5,584,873, all incorporated herein by reference. These patents and U.S. Pat. Nos. 4,640,983, 4,964,414, 5,246,014, 5,483,022, and 5,760,341, all incorporated herein by reference, present a number of alternative designs of such stranded filament wires or cables. As shown therein, the stranded filaments are formed of highly conductive alloys and used in small diameter lead bodies in either a straight configuration or a in coiled configuration. When straight conductors are employed, it is necessary to resort to use of an introducer rather than the stylet to pass the lead through the vessel paths identified above and to position and fix the distal electrode of the lead at the desired site in the heart chamber. In the coiled configuration, a plurality of stranded filament conductors are wound into a like plurality of intertwined, parallel wound, coils that are electrically connected together in a redundant manner as disclosed in the above-incorporated '983 and '022 patents.
In these patents, complex cables are formed of a number of filaments of single alloy material or of filaments formed with an inner core of one material e.g., silver or stainless steel, and an outer sheath of another material, e.g., MP35N, using the DBS or DFT extrusion techniques. The current carrying capacity of cardioversion/defibrillation lead conductors formed in these ways is maximized for the cross-section dimensions of the filaments and cables.
These efforts to minimize lead body diameter, maximize lead body flexibility, the number of separate conductors encased in the lead body, the current carrying capacity of each such conductor have to be balanced by retaining adequate resistance to fracture. The transvenous pathway can include a number of twists and turns, and the lead body can be forced against bony structures of the body that apply stress to It, causing the lead body to be crushed and/or causing the lead conductor to break. Moreover, the heart beats approximately 100,000 times per day or over 30 million times a year, and each beat stresses at least the distal portion of the lead body located within a heart chamber or cardiac vessel. Over the years of implantation, the lead conductors and insulation are subjected to such cumulative mechanical stresses that can result in degradation of the insulation or fractures of the lead conductors with untoward effects on device performance and patient well being.
For, example, the percutaneous subclavian, venipuncture approach is commonly employed in the implantation of endocardial pacing leads or cardioversion/defibrillation leads, and it involves passing the lead body through the costoclavicular region where it can be crushed as described in U.S. Pat. No. 5,545,203, incorporated herein by reference. Efforts have been undertaken to improve the crush resistance of lead bodies that encase a plurality of straight or coiled lead conductors as described in the '203 patent or in commonly assigned U.S. Pat. No. 5,584,873, incorporated herein by reference.
However, despite these improvements, not all causes of lead fracture can be overcome by lead body design, and lead conductor fractures still occur from time to time. Fractures that occur in multi-filar or stranded filament lead conductor wires or cables, typically commence with less than all of the filaments. Lead conductivity may decrease as a result, and the reduced conductivity affects sensing and current stimulation efficacy. Moreover, the broken wire or filament ends may make intermittent contact, resulting in erratic conductivity changes.
The resulting system performance deteriorates, but the drop in performance may go unnoticed for a time until a complete break occurs. When system performance deterioration becomes symptomatic, it is often difficult to determine that it is due to a lead conductor fracture. The lead conductor wires or filaments or a cable are so small that it is typically not possible to visualize the break under fluoroscopy, although complete separations of the broken ends of a lead conductor have been observed in this manner. But, such complete separations usually follow earlier conductor fractures, and it is much preferable to be able to diagnose a lead conductor fracture at the earlier stages and to replace the lead before the patient is endangered.